1. Field of the Invention
Embodiments of the invention relate to nonvolatile memory devices. More particularly, embodiments of the invention relate to an apparatus and operating method of a nonvolatile memory device having three-level nonvolatile memory cells.
2. Discussion of Related Art
Next generation nonvolatile memory devices are being developed for use in portable consumer products to provide high capacity and low power consumption. While dynamic RAM (DRAM) and flash memory devices store data by using charge, nonvolatile memory devices utilize resistance material to store data by changing the state of phase-change material. These memory devices include, for example, PRAMs (Phase change Random Access Memory) utilizing phase-change material such as a chalcogenide alloy that can be switched between two states, RRAMs (Resistance Random Access Memory) employing material having a variable resistance characteristic of complex metal oxides, and MRAMs (Magnetic Random Access Memory) utilizing the resistance change of MTJ (Magnetic Tunnel Junction) thin films according to the magnetization state of a ferromagnetic substance. The resistance value is maintained in these devices even when no current or voltage is supplied demonstrating nonvolatile memory characteristics.
Various methods for storing as many bits as possible in a limited wafer space have also been developed. For example, one possible way of fabricating even more nonvolatile memory cells in a limited wafer area is to develop and use more sophisticated lithography methods and apparatuses. Alternatively, the integration degree of a nonvolatile memory device can be increased by storing more than one bit in a memory cell (hereinafter referred to as a multi-level nonvolatile semiconductor memory device).